Porous membrane filtration systems require regular backwashing of the membranes to maintain filtration efficiency and flux while reducing transmembrane pressure (TMP) which rises as the membrane pores become clogged with impurities. Such systems usually have a number of membranes in the form of bundles or arrays of membranes located in a feed tank. Feed is thus applied to the surfaces of the membranes and filtrate withdrawn from the membrane lumens. Typically, during the backwash cycle the impurities are forced out of the membrane pores by pressurised gas, liquid or both into the feed tank or cell. The liquid containing impurities and deposits from the membranes is then drained or flushed from the tank.
The waste liquid displaced from the tank needs to be disposed of or reprocessed, usually in an environmentally safe manner, so any reduction in the volume of such waste liquid is seen as advantageous in terms of environmental impact and cost.
The draining or flushing of the tank, particularly when large arrays of membranes are used also requires time which results in down time of the filtration cycle. In order to reduce this down time large pumping systems are required to quickly drain and refill the tank. Where tanks or cells are arranged in banks and feed is used to refill the tank, a lowering in levels in other cells may be produced during the refill process. This again impinges on operating efficiency of the filtration system.
Reduction in backwash volume also reduces the volume of chemical cleaning agents required in some systems. This has the two-fold advantage of reducing cost in terms of chemical requirements while also reducing waste disposal problems.
Many filtration systems also employ a gas bubble aeration method to assist in scouring unwanted deposits from the surfaces of the porous membranes during the cleaning stage. In order for such aeration to be effective the membranes must be fully immersed in liquid.
Typically the cleaning p in such systems includes ceasing flow of feed liquid to the vessel in which the membranes are located, continuing the filtration process until the level of liquid in the tank reaches the top of the membranes, aerating the membranes with gas bubbles to scour impurities from the outer membrane surfaces for a period of time, then liquid backwashing the membrane pores. The tank is then drained or flushed to remove the waste liquid containing the impurities dislodged during the aeration and backwashing processes.
It will be appreciated that it is necessary to keep the membranes submerged so as to make the aeration step effective. Adding the liquid backwash at the end of the aeration step results in the liquid level in the tank rising above the membranes. This is “wasted volume” in terms of the requirement of keeping the membranes submerged.